A neoplasm arising from the epithelial cells of the thymus. Although thymomas are usually encapsulated tumors, they may invade the capsule and infiltrate the surrounding tissues or even metastasize to distant anatomic sites. The following morphologic subtypes are currently recognized: type A, type B, type AB, and type C. Thymomas type B are further subdivided into types B1, B2, and B3. Thymoma types B3 and C usually have the most aggressive clinical course. --2003